As technologies in electronics have been remarkably evolved, a variety of electronic equipment has become smaller and lighter. Accordingly, it has been required that batteries as portable power sources be increasingly smaller, lighter, and higher in energy density.
Conventionally, aqueous solution type secondary batteries, such as lead batteries and nickel-cadmium batteries, are primarily used as secondary batteries for general use. However, though these aqueous solution type secondary batteries exhibit excellent cyclic properties, they are not satisfactory in weight and energy density.
Meanwhile, recently, a non-aqueous liquid electrolyte secondary battery using lithium or a lithium alloy for its anode has been researched and developed prevalently. This non-aqueous liquid electrolyte secondary battery has a high energy density, exhibits only a small amount of self-discharge, and is lightweight. However, in the non-aqueous liquid electrolyte secondary battery, lithium is crystallized in dendritic form in proceedings of charge/discharge cycles, and the crystallized lithium finally reaches the cathode to generate an internal short circuit. Therefore, it is difficult to employ this non-aqueous liquid electrolyte secondary battery for practical use.
On the contrary, a lithium ion based non-aqueous liquid electrolyte secondary battery which uses a carbon material for its anode has attracted attention for the following reasons. The non-aqueous liquid electrolyte secondary battery utilizes dope and undope of lithium into and from between carbon layers. By doing so, the battery does not exhibit the dendritic precipitation of lithium in the charge even though charge/discharge cycles proceed. The battery exhibits satisfactory charge/discharge cyclic properties.
Meanwhile, the carbon materials are roughly categorized into low-crystalline carbon materials, such as cokes and glass like carbon, having a pseudo-graphite structure or a turbostratic structure, and high-crystalline carbon materials, such as graphite, having a grown crystalline structure.
Among these carbon materials, the low-crystalline carbon material has conventionally been used as the anode material of the non-aqueous liquid electrolyte secondary battery, because of its high compatibility with propylene carbonate (PC) normally used for the non-aqueous solvent of the non-aqueous liquid electrolyte secondary battery.
That is, both the non-aqueous liquid electrolyte secondary battery using metallic lithium for the anode and the non-aqueous liquid electrolyte secondary battery using a carbon material for the anode have conventionally employed PC as the main component of the non-aqueous solvent of the liquid electrolyte. Using PC for the non-aqueous solvent is advantageous in that it forms a film stable with metallic lithium, particularly in the non-aqueous liquid electrolyte secondary battery using metallic lithium for the anode.
If the non-aqueous liquid electrolyte secondary battery mainly using PC for the non-aqueous solvent has the anode formed of the low-crystalline carbon material, a practical charge/discharge capacity can be obtained. However, if the battery has the anode formed of the high-crystalline carbon material, the anode is not sufficiently doped with lithium and the practical charge/discharge capacity cannot be obtained. A conceivable reason therefor is that, with the anode formed only of the high-crystalline carbon material, as PC decomposes on the anode surface to generate a propylene gas and form lithium carbonate, the capacity is consumed in the reaction, disturbing the lithium dope into the anode, as reported in A. N. Dey and B. P. Sullivan, J. Electrochem. Soc., Vol. 117, 1970, page 222.
Actually, however, the high-crystalline carbon material has a higher true density than the low-crystalline carbon material. Therefore, in order to assure a satisfactory electrode packing property to obtain a high energy density, it is much more advantageous to use the high-crystalline carbon material as the anode material.
Thus, to make such a high-crystalline carbon material usable, a mixed solvent of PC and ethylene carbonate (EC) is reported in J. Electrochem. Soc., Vol. 137, No. 7, 1990, page 2009. According to this report, by using the mixed solvent of PC and EC, it is possible to carry out charge/discharge with the anode formed of a graphite material.
However, since this mixed solvent has only poor wettability with a porous polypropylene film used in a practical battery and also has a high viscosity, it does not exhibit satisfactory properties.
Since non-aqueous solvents suitable for the high-crystalline carbon material have not been found as described above, the anode performance of the high-crystalline carbon material is not currently considered. However, as a higher energy density is being demanded of the non-aqueous liquid electrolyte secondary battery, it is critical to sufficiently utilize the properties of the high-crystalline carbon material.